Transverse Wave
Physics 10
Vibrations and Waves
disturbance
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direction of travel
Longitudinal Wave
disturbance
direction of travel
If we double the frequency of a wave,
what happens to its period? What
happens to the wave speed?
⇒ Frequency and period are reciprocals of
one another: f = 1/T and T = 1/f. Double one
and the other is half as much. So doubling
the frequency of a wave halves the period.
The waves speed does not change, it only
depends upon the properties of the medium.
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Interference
Interference
⇒ If two waves exist at
the same point in space
at the same time, they
will interfere with each
other.
Superposition Principle: When two or more waves are
present simultaneously at the same place, the resultant disturbance is
the sum of the disturbances from the individual waves.
animations
Interference
Superposition Principle for Waves
Constructive
Interference
Destructive
Interference
⇒ Overlapping waves do not alter the travel of the
other wave.
⇒ The individual waves move independently
of one another.
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Reflections at a Boundary
⇒ When a wave hits a hard boundary like a wall that
is too rigid to shake, the wave will reflect back.
Standing Waves
antinodes
⇒ If we continue producing waves, the incident
waves and the reflected waves will interfere with each
other.
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Standing Waves
Standing Waves
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The Doppler Effect
The Doppler Effect
⇒ Imagine you have a bug bobbing up and down in
⇒ If the bug is moving as it bobs up and down, then in
the middle of a quiet puddle of water. The frequency
of the waves it produces would be the same as the
frequency the bug is bobbing up and down.
effect, it chases part of the waves it has produced.
A
B
⇒ An observer at points A or B would detect waves of
What happens if the bug is moving?
a different frequency even though the bug still bobs up
and down at the same frequency.
The Doppler Effect
The Doppler Effect
⇒ If the bug is moving toward an observer, the
observer will measure a higher frequency.
⇒ If the bug is moving away from an observer, the
observer will measure a lower frequency.
I measure a
lower frequency
I measure a
higher frequency
Definition: the change in frequency
measured by an observer because either
the source of the waves or the receiver
is moving.
⇒ The is why you hear the pitch
(frequency) of a car horn change when
it passes you.
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The Doppler Effect
Exercise 25
Why is there a Doppler effect when the source of sound
is stationary and the listener is in motion? In which
direction should the listener move to hear a higher
frequency? A lower frequency?
⇒ The frequency of sound (pitch) increases when
a source moves towards you.
⇒ The frequency of sound (pitch) decreases when
a source moves away from you.
The Doppler Effect
⇒ The Doppler effect is a change in frequency that
occurs as a result of the motion of the source, the
receiver, or both. If you move toward a stationary sound
source, you encounter the waves more frequently and the
you hear a sound of higher frequency. If you move away
from the source, you encounter the waves less frequently
and you hear a sound of lower frequency.
Bow Waves and Shock Waves
⇒ If the bug is moving at the same speed as the
waves it produces, the wave fronts pile up in
front of the bug. A bow wave is formed!
⇒When an observer is moving towards a stationary source, the
frequency is greater (and the wavelength is smaller).
⇒When an observer is moving away from a stationary source, the
frequency is smaller (and the wavelength is bigger).
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Bow Waves and Shock Waves
Breaking the Sound Barrier
bow wave ⇒ V-shaped wave produced by an object
moving on a liquid surface faster than the wave speed.
⇒ A similar thing happens when a plane is traveling
faster than the speed of sound. A sonic boom is heard
when the shock wave reaches listeners on the ground.
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